The planet Mars
has always captured our imagination. In ancient times, its wandering motion
and blood-red color were portents of ill tidings and earned it the name
of the God of War. Its looping path against the background stars vexed
Ptolomy and other Sun-centrists, inspired Copernicus to place the Sun
at the center of the solar system, and spurred Kepler to discover his
laws of motion. Early telescopic observations revealed surface markings
that changed with time; wishful thinking turned those variabilities into
evidence of a vast martian civilization. We know better today, but little
green men from Mars remain a staple in pulp science fiction and in tabloid
sensationalism.

Evidence of martian winds
can be seen with difficulty in the telescope. McLaughlin, in 1954, was
the first to speculate that periodic albedo changes on Mars might be due
to wind action rather than changes in vegetation. Early space probes viewed
only a limited section of the heavily cratered part of the martian surface,
and some investigators drew the conclusion that all of Mars was like our
Moon and, hence, uninteresting. Others predicted that winds and dust in
the atmosphere might prove significant. The breakthrough came in 1971
with the arrival of the Mariner 9 spacecraft in Mars orbit. A global dust
storm was raging and the entire surface was obscured, but as the storm
slowly abated, an astonishing variety of geologic features was revealed.
Martian science was forever changed and an important lesson was learned:
Don't think you know everything about a planet based on what you see on
a small portion of its surface! In 1976, Vikings 1 and 2 arrived at Mars,
each having a lander module and an orbiting instrument platform. They
were phenomenally successful; most of what we know about Mars comes from
the Viking mission. We are still analyzing the results more than a decade
later, and there remains much to be done.

The slides in this
set were selected to provide an overview of the types of aeolian activity
and landforms found on Mars. It is by no means all-inclusive; thousands
of Mariner 9 and Viking photographs would have to be shown. There is a
glossary of martian placenames and terms
after the slide descriptions. Not cited are the hundreds of scientific
journal articles that deal with the topic of martian aeolian activity
and landforms; however, there is included a short reference
list of some of the general texts on the topic of Mars. Many pertinent
references are cited therein if you are inclined to pursue a more in-depth
study of this fascinating topic.

Section
1. Aeolian Activity (slides 1–5)
Modification of the martian surface by the wind is a process occurring
presently, as evidenced by atmospheric dust storms, dust devils, and perhaps
even tornados.

Section
2. Wind Streaks (slides 6–9
Wind streaks are perhaps the most striking and widespread aeolian feature
on Mars. Some have been observed to undergo changes in short amounts of
time, indicating that the aeolian erosion and/or deposition that formed
them is presently active.

Section
3. North Circumpolar Dunefiels (slides 10–14)
Encircling the northern polar cap of Mars is a large dunefield, or erg.
The composition and source of the material in the dunefield have not been
positively determined. Located within it are transverse dunes of many
types.

Section
4. Other Dunes (slides 15–16)
The north circumpolar erg is not the only place dunes are found on Mars.
Further examples are shown in the next two slides. Very few martian dunes
are found in the open; most dunes are “trapped” in some way
by local topography. The odds of finding an isolated dune are low because
the dunes migrate very quickly until they get trapped by an obstacle of
some sort, where they can remain for a long time.

Section
5. Wind Erosion Features (slides 17–19)
There are other prominent wind erosion features on Mars in addition to
the dark streaks. On Earth, when erodible rocks and sediments are exposed
to a strong unidirectional wind, they are sculpted into streamlined shapes
that have been likened to inverted boat hulls. These wind-shaped hills
are called yardangs, a term derived from the Turkistani word yar, meaning
steep bank. Terrestrial yardangs range in size from a few meters to tens
of kilometers and are best developed in arid areas where they would not
be destroyed by running water.

Section
6. Laminated Polar Terrain and Other Aeolian-Related Features
(slides 20–24)
The polar caps of Mars can be seen with a small telescope and their seasonal
changes have been observed for hundreds of years. The part of the cap
that varies is frozen carbon dioxide (“dry” ice). A small
residual cap remains throughout the year; the larger northern cap is water
ice, the southern is probably also water ice, but with a frosting of dry
ice. Beneath the ice are thick sedimentary deposits: in the south, atop
heavily cratered terrain, in the north, on top of plains units.

One of the most startling
observations made by Mariner 9 was of the southern residual polar cap
and the deposit beneath it, which was found to be composed of thin layers
or laminations. The layering is thought to be due to cyclical climatic
variations. The individual layers are too thick to be due to annual changes
in climate (Mars has seasons just like the Earth, but they are more severe
because the orbit of Mars is more eccentric than that of Earth and the
martian atmosphere is too thin to prevent large temperature swings). However,
the laminations are thin enough to be related to climate changes caused
by naturally occurring cyclical variations in the orientation of Mars’
axis of rotation, akin to a mechanism proposed by Milankovitch as a trigger
for terrestrial ice ages.